Mass storage devices are employed in a variety of applications where large amounts of information need to be stored in a retrievable manner. Such applications include computers and computer-type applications, wherein one or more mass data storage devices, often referred to as hard disk drives, CD-ROMs, or the like, have one or more rotating disks in which data can be stored. For example, disks in a typical hard disk drive comprise a magnetic, optical, or other media that can store such data, and from which such information can be read or retrieved. Data or other information is written to or recorded in certain field portions of rings or tracks that are physically located progressively radially outwardly from the center of the disk. Such disks are often divided into radial tracks, wherein multiple sectors are formed within individual tracks on the disk.
A read/write head is provided which is scanned above the disk surface in a controlled fashion while the disk is rotated, so as to electrically interface with particular tracks and sectors of the disk in read or write operations. The read/write head is part of a read channel in the mass storage device, which interfaces the computer with the storage disk. The disk may be used to store many types of information or data, including user data as well as control information, used to position the read/write head at the desired location relative to the rotating disk. Such data may be stored in segmented locations or regions on the disk, such as user sectors and control or servo sectors for storing position information used in positioning the read/write head. For example, the data on a disk may include servo data, such as Gray code information, automatic gain control (AGC) signals, head alignment bursts, and the like, recorded in servo sectors, as well as user data, recorded in user data sectors.
Each track on the disk generally includes one or more servo sectors located at spaced locations along the track. Each servo sector has a number of fields, each for providing information for location or control of the head data transducer. For instance, an AGC burst field is provided, which enables AGC circuitry to automatically adjust the gain in the head amplifiers to allow the subsequent data to be properly detected. Also included in the servo sector is a field having one or more sync marks so that the longitudinal position of the head relative to the track of interest can be determined, which may follow the AGC field. The sync marks may be used, for example, to enable subsequent fields, such as the user data sectors or Gray code data to be located by counting a predetermined elapsed time from the time that the sync marks are detected.
A Gray code field is also provided in the sector, having Gray code data therein from which the identification of the particular radial track over which the head is positioned can be established. Following the Gray code field is a binary data field, for example, having longitudinal track identification information, so that the identity of each track region between adjacent servo sectors can be established. After the binary data field, a number of servo burst fields are provided for precision alignment of the head laterally with respect to the selected track.
In a read operation, one or more read/write heads are selectively radially moved over the track which includes the data of interest. Gray codes prerecorded onto each data track or ring are decoded to determine the instantaneous position of the data transducer heads with respect to the rotating disk. The read/write transducer heads are typically positioned by means of a closed-loop servo system in accordance with the decoded Gray code that has been detected. More particularly, the data transducer heads read the Gray code servo information recorded within data tracks on disks. The servo information typically includes track addresses, and optionally sector addresses and servo bursts. The track addresses are used as coarse positioning information and servo bursts are used as fine positioning information.
As the transducer heads are being moved to a desired track location, the transducer head reads the track addresses provided by the Gray codes in order to determine its instantaneous location. Often, the transducer head is positioned between two adjacent tracks, and may receive a superposition of signals from both tracks. However, due to the data characteristics of Gray codes, the position ambiguity can be resolved. Thus, when the head is on an interface between two tracks, either of the two track addresses will be correctly detected, due to the characteristics of the Gray code used. The Gray codes may then be used to reposition the head radially so as to no longer be on the interface between two tracks.
The data sectors on the selected track may be synchronously recovered after timing acquisition by a phase lock loop circuit, but the detection of the servo sectors on a track are often performed asynchronously. It is difficult to realize high-speed detection and high-density recording by asynchronous servo detection methods. Various synchronous servo techniques have accordingly been employed, such as partial response maximum likelihood (PRML) signal processing. In this approach, timing is synchronized in the servo preamble region by a phase lock loop circuit, and the track address and servo bursts are synchronously sampled and decoded.
Partial response processing is thus employed in order to address intersymbol interference (ISI). However, as data densities increase in mass storage devices, adjacent channel responses to transitions in media tend to interact with each other such that the ideal single transition shape is degraded randomly, leading to difficulties in considering the transition shape as an appropriate transition symbol at detector stages. Where the partial channel response takes the form of linear superposition of known individual symbol shapes, interference between adjacent transitions can be anticipated and taken into consideration in detector strategies. Typical read channels for such mass storage devices thus provide equalization of the response channel to a standard shape and a Viterbi maximum likelihood detector. Equalization addresses ISI control by placing the sampling moments in a position on the response shape, so as to control interference. The Viterbi detector analyzes the received signal shape, based on an appropriate succession of samples from which a decision can be inferred.
Partial response channels coupled with appropriate detectors thus facilitate increased density in data storage devices, particularly as data densities continue to increase. A polynomial operator P characterizes the partial response channel which applies to a non-return-to-zero (NRZ) random initial binary sequence via polarization and converts the binary sequence into a ternary sequence, which is then forwarded to the detector input. Typical channels have a (1−D) polynomial characteristic to model differential action of the media-head interaction, with a single sample in the center of the received symbol. The partial response 4 (PR4), is a first partial response applied in mass storage devices, having a (1−D)(1+D) polynomial characteristic, wherein the (1+D) factor designs the two symmetric samples on the equalized symbol response (1,1 sequence). Increasing 1+D factors to 2 in the P expression results in an EPR4 (Extended Partial Response 4) channel, with three samples per symbol (two symmetric ½ amplitude samples and one central full amplitude sample (−1,2,1 sequence), and in E2PR4 for (1−D)(1+D)3, which has two unequal peers of symmetric samples in 1,3,3,1 sequence. The number and size of samples per symbol fix the accepted interference, to be taken into consideration at the decoding stage.
Mass storage device manufacturers continue to strive for greater capacity (e.g., higher data density) in hard disk drives and other mass storage devices. However, as a result, interference between adjacent data symbols (ISI) has increased, lowering the signal-to-noise ratio in the detected signals from the data storage medium. Thus, as data density is increased, it is more difficult to properly detect the signals read from the data medium, and consequently, more difficult to rapidly and properly position the data read/write head transducers. Therefore, there remains a need for improved mass storage device read channels and Gray code detectors therefor, by which servo data can be properly read from high density data storage disks for servo positioning of read/write heads.